1. Field
Embodiments relate to a polymer and an organic photoelectric device including the same.
2. Description of the Related Art
An organic photoelectric device has been highlighted as the next-generation display device. The organic photoelectric device may be driven at a low voltage, and may afford advantages not provided by a liquid crystal display (LCD), e.g., a small thickness, a wide viewing angle, and a rapid response speed. The organic photoelectric device of a middle size or less may also provide equivalent or better image quality to a liquid crystal display (LCD) compared to other displays, and its manufacturing process is very simple. Therefore, it is predicted to be advantageous in terms of cost in the future.
An organic photoelectric device may include an organic light emitting material between a rear plate including ITO transparent electrode patterns as an anode on a transparent glass substrate and an upper plate including a metal electrode as a cathode on a substrate. When a predetermined voltage is applied between the transparent electrode and the metal electrode, current flows through the organic light emitting material to emit light.
In 1987, Eastman Kodak, Inc., developed an organic light emitting diode including a low molecular weight aromatic diamine and an aluminum complex as an emission-layer-forming material (Applied Physics Letters. 51, 913, 1987). C. W. Tang et al. disclosed a practicable device as an organic light emitting diode in 1987 (Applied Physics Letters, 51 12, 913-915, 1987).
The organic layer may have a structure in which a thin film (hole transport layer (HTL)) of a diamine derivative and a thin film of tris(8-hydroxy-quinolate)aluminum (Alq3) are laminated. The Alq3 thin film of Alq3 functions an emission layer for transporting electrons.
Generally, an organic photoelectric device is composed of an anode of a transparent electrode, an organic thin layer of a light emitting region, and a metal electrode (cathode) formed on a glass substrate, in that order. The organic thin layer may include an emission layer, a hole injection layer (HIL), a hole transport layer (HTL), an electron transport layer (ETL), or an electron injection layer (EIL). It may further include an electron inhibition layer or a hole inhibition layer due to the emission characteristics of the emission layer.
When an electric field is applied to the organic light emitting diode, the holes and electrons are injected from the anode and the cathode, respectively. The injected holes and electrons are recombined on the emission layer though the hole transport layer (HTL) and the electron transport layer (ETL) to provide light emitting excitons. The provided light emitting excitons emit light by transiting to the ground state.
The light emission may be classified as a fluorescent material including singlet excitons and a phosphorescent material including triplet excitons according to the light emitting mechanism.
The phosphorescent light emitting material may be useful as a light emitting material (D. F. O'Brien et al., Applied Physics Letters, 74 3, 442-444, 1999; M. A. Baldo et al., Applied Physics letters, 75 1, 4-6, 1999). Such phosphorescent emission occurs by transition of electrons from the ground state to the exited state, non-radiative transition of a singlet exciton to a triplet exciton through intersystem crossing, and transition of the triplet exciton to the ground state to emit light.
When the triplet exciton transitions, it cannot directly transition to the ground state. Therefore, the electron spin is flipped, and then it transitions to the ground state. Thus, it provides a characteristic of extended lifetime (emission duration) relative to that of fluorescent emission.
In other words, the duration of fluorescent emission is extremely short (at several nanoseconds), but the duration of phosphorescent emission is relatively long (such as at several microseconds), so that phosphorescent emission provides a characteristic of extending the lifetime (emission duration) to more than that of the fluorescent emission.
Quantum mechanically, when holes injected from the anode are recombined with electrons injected from the cathode to provide light emitting excitons, the singlet and the triplet are produced in a ratio of 1:3, in which the triplet light emitting excitons are produced at three times the amount of the singlet light emitting excitons in the organic light emitting diode.
Accordingly, the percentage of the singlet exited state is 25% (the triplet is 75%) in the case of a fluorescent material, so it has limits in luminous efficiency. On the other hand, in the case of a phosphorescent material, it can utilize 75% of the triplet exited state and 25% of the singlet exited state, so theoretically the internal quantum efficiency can reach up to 100%. When a phosphorescent light emitting material is used, it has advantages in an increase in luminous efficiency of around four times that of the fluorescent light emitting material.
In the above-mentioned organic photoelectric device, a light emitting colorant (dopant) may be added in an emission layer (host) in order to increase the efficiency and stability in the emission state.
In the above-mentioned organic light emitting diode, a light emitting colorant (dopant) may be added in an emission layer (host) in order to increase the efficiency and stability in the emission state. In this structure, the efficiency and properties of the light emission diodes are dependent on the host material in the emission layer.